Many devices are well-known in the prior art for measuring the capacity or remaining charge of various types of storage batteries. For batteries having a liquid electrolyte, an hydrometer has found application since it provides a display of specific gravity of the electrolyte, which is generally proportional to remaining charge. However, measurement of specific gravity is of limited usefulness because the density of the electrolyte near the top of the cell frequently differs from the density near the plates. In addition, diffusion must take place over time, and thus as much as twenty-four hours may be required for accurate measurement using this approach. Still further, the temperature of the electrolyte must be known for accurate measurement.
Other devices for measuring storage capacity have involved measurement of battery terminal voltages, for example using an expanded scale near the top of the voltage range. However, voltage is dependent upon the state of the electrolyte, and thus this approach suffers from each of the limitations inherent in monitoring the electrolyte itself. In addition, voltage at the battery terminals must be measured under no load/no charge conditions, and thus requires that all charging or load circuits be disconnected during measurement. If such circuits cannot be disconnected, battery impedance must be considered, which results in complex compensation circuits. Wattmeters are subject to similar limitations.